Blackmagic Design - Infrared Filtration

All of our current Video Aliasing Filters for Blackmagic cameras now include very effective infrared filtration as an integral part of their optical designs. This profoundly corrects the Blackmagic cameras' color rendering for dark and especially black objects: These cameras are well known for excessive sensitivity to near-infrared light, which can often give a muddy, brownish cast to fabrics and other items which visually have a clean black coloration.

The following video frames were shot in bright, mid-day sunlight with a Blackmagic Micro Cinema Camera and eight stops of ND filtration - using inexpensive generic front-of-the-lens ND filters.

All three items of clothing were newly purchased for this test. Visually in sunlight, all three appear a dense, neutral black.

However, the sweatshirt on the right, and the pants on the left, have been dyed black using common commercial organic dyes that actually reflect a significant amount of infrared light.

Our eyes don't see this infrared light - so these items look black to the human visual system. But the Blackmagic cameras do see it. This infrared light is detected mainly by the camera's red channel, giving these blacks a brownish, muddy hue.

Supplementary infrared filtration can block this excess infrared light from reaching the image sensor. Compare a frame shot a few minutes later using the Mosaic VAF/IR-BMD-MFT filter: This filter is effectively blocking infrared wavelengths that do not contribute to human visual color perception - so the blacks as seen by the camera take on their natural, dense, dark neutral tones.

Transmission Spectrum

Many Cinematographers using the Ursa Mini cameras have a high level of interest in the particulars of infrared filtration, and the impacts that our VAF/IR filter may have on Ursa Mini color rendering. Therefore, in response to several requests, we supply the following design curve for the VAF/IR filter spectral transmission:

As can be seen by this graph, the spectral transmission of the filter is very flat throughout the band of wavelengths contributing to human color perception. (The human visual system has negligible sensitivity outside approximately the 410‑680 nm spectral window.) It is for this reason, in combination with the filter's strong suppression of the near‑IR, that the filter's IR filtration has negligible impact on inherent camera color rendering.

Addendum

Please note that the video frames shown above were taken outdoors in full, bright sunlight - but on a windy day with very thin, high clouds. There was significant, minute-to-minute variation in the natural lighting, and this has contributed to some of the differences visible between these frames - for example the changes in shadow contrast and in specular leaf shine.

We observe also, that the foliage in this scene itself reflects a significant amount of infrared. This gives the foliage in the image taken by the stock Blackmagic camera, a warmish and slightly yellow cast. In contrast, the second image using effective infrared filtration renders this foliage with a cooler and more bluish coloration - which in fact is much more faithful to the foliage's true visual appearance.